The subject invention is directed generally to built-in test circuitry for allowing AC or delay testing of digital integrated circuits, and more particularly to built-in test circuitry for allowing AC testing without the need for high bandwidth test equipment.
Digital integrated circuits are typically designed to operate at predetermined clock rates, and AC or delay testing is performed to ascertain whether a digital integrated circuit is capable of operating at its design clock rate. As is well known, the maximum allowable clock rate of a digital integrated circuit is determined by the propagation delays of the combinatorial logic networks between latches or registers. Since internal nodes in integrated circuits cannot be directly accessed, the delays of individual gates cannot be directly measured for speed testing.
Testing of an integrated circuit is typically performed by use of automatic test equipment (ATE) which applies data and clock signals (commonly called test vectors) to the inputs of the integrated circuit, samples the resulting integrated circuit outputs, and compares the outputs with expected values. For AC or speed testing, the input data rate and clock frequency must be at the maximum for which the integrated circuit was designed, and the outputs must be sampled at such maximum rate.
A consideration with direct AC measurements by ATE is the requirement for high accuracy measuring capability. Testers meeting such requirement for high pin count and high clock rates are very expensive. Moreover, the fixturing (i.e., the interconnection components between the ATE and the integrated circuit being tested) required for ATE AC testing at high clock rates is expensive due to the higher bandwidth requirements, and also severely limits accuracy, particularly for wafer probe wherein microscopically aligned probes are placed in contact with the input and output (I/O) pads of the integrated circuits on a wafer. The connections to the wafer probes tend to limit bandwidth, and thus the data rate and clock frequency that can be used for testing.